The present invention relates to fuel cells operating directly on fuel streams comprising dimethyl ether in which dimethyl ether is directly oxidized at the anode. In particular, it relates to solid polymer fuel cells operating directly on liquid fuel streams comprising dimethyl ether. The dimethyl ether may serve as the primary fuel or as a component of a mixed fuel.
Solid polymer electrochemical fuel cells convert reactants, namely fuel and oxidants, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. An electrocatalyst is needed to induce the desired electrochemical reactions at the electrodes. Solid polymer fuel cells operate in a range from about 80xc2x0 C. to about 200xc2x0 C. and are particularly preferred for portable and motive applications. Solid polymer fuel cells employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d) which comprises a solid polymer electrolyte or ion-exchange membrane disposed between the two electrode layers. Flow field plates for directing the reactants across one surface of each electrode substrate are generally disposed on each side of the MEA. The electrocatalyst used may be a metal black, an alloy or a supported metal catalyst, for example, platinum on carbon. The electrocatalyst is typically incorporated at the electrode/electrolyte interfaces. This can be accomplished, for example, by depositing it on a porous electrically conductive sheet material, or xe2x80x9celectrode substratexe2x80x9d, or on the membrane electrolyte.
Effective sites on the electrocatalyst are accessible to the reactant, are electrically connected to the fuel cell current collectors, and are ionically connected to the fuel cell electrolyte. Electrons, protons, and possibly other species are typically generated at the anode electrocatalyst. The electrolyte is typically a proton conductor, and protons generated at the anode electrocatalyst migrate through the electrolyte to the cathode.
A measure of electrochemical fuel cell performance is the voltage output from the cell for a given current density. Higher performance is associated with a higher voltage output for a given current density or higher current density for a given voltage output. Another measure of fuel cell performance is the Faradaic efficiency, which is the ratio of the actual output current to the total current associated with the consumption of fuel in the fuel cell. For various reasons, fuel can be consumed in fuel cells without generating an output current, such as when an oxygen bleed is used in the fuel stream (for removing carbon monoxide impurity) or when fuel crosses through a membrane electrolyte and reacts on the cathode instead. A higher Faradaic efficiency thus represents a more efficient use of fuel.
A broad range of reactants have been contemplated for use in electrochemical fuel cells and such reactants may be delivered in gaseous or liquid streams. The oxidant may, for example, be substantially pure oxygen or a dilute oxygen stream such as air. The fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream derived from a suitable feedstock, or a suitable gaseous or liquid organic fuel mixture.
The choice of fuel may vary depending on the fuel cell application. Preferably, the fuel is relatively reactive electrochemically, inexpensive, easy to handle, and relatively safe for the environment. Hydrogen gas is a preferred fuel since it is electrochemically reactive and the by-products of the fuel cell reaction are simply heat and water. However, hydrogen can be more difficult to store and handle than other fuels or fuel feedstocks, particularly in non-stationary applications (e.g. portable or motive). For this reason, liquid fuels are preferred in many applications. Fuel cell systems employing liquid fuels generally incorporate a reformer to generate hydrogen as required from a liquid feedstock that is easier to store and handle, e.g. methanol. However, the use of a reformer complicates the construction of the system and results in a substantial loss in system efficiency. To avoid using a separate reformer, fuels other than hydrogen may instead be used directly in fuel cells (i.e. supplied unreformed to the fuel cell anodes). Inside the fuel cell, a fuel mixture may be reacted electrochemically (directly oxidized) to generate electricity or instead it may first be reformed in-situ (internally reformed), as in certain high temperature fuel cells (e.g. solid oxide fuel cells). After being internally reformed, the fuel is then electrochemically converted to generate electricity. While such fuel cell systems may employ fuels that are easier to handle than hydrogen, without the need for a separate reformer subsystem, generally hydrogen offers fundamental advantages with regards to performance and the environment. Thus, improvements in these areas are desirable in order for internally reforming and direct oxidation fuel cell systems to compete more favorably to hydrogen-based systems.
A direct methanol fuel cell (DMFC) is a type of direct oxidation fuel cell that has received much attention recently. A DMFC is generally a liquid feed solid polymer fuel cell that operates directly on an aqueous methanol fuel mixture. There is often a problem in DMFCs with substantial crossover of methanol fuel from the anode to the cathode side through the membrane electrolyte. The methanol that crosses over then reacts with oxidant at the cathode and cannot be recovered, resulting in significant fuel inefficiency and deterioration in fuel cell performance. To reduce crossover, very dilute solutions of methanol (e.g. about 5% methanol in water) are typically used as fuel streams in DMFCs. Unfortunately, such dilute solutions afford only minimal protection against freezing during system shutdown in cold weather conditions, typically down to about xe2x88x925xc2x0 C.
In PCT/International Publication No. WO 96/12317 (Application No. PCT/US94/11911), alternative liquid fuels, including dimethoxymethane (DMM), trimethoxymethane (TMM), and trioxane, are suggested for direct use in liquid feed solid polymer fuel cells. Like methanol, these fuels can be oxidized at the fuel cell anode to form carbon dioxide and water at a rate that provides satisfactory fuel cell performance. Methanol appears to be an intermediate product of the oxidation for each of these fuels.
Dimethyl ether (DME) is available in quantity and has been considered as a cleaner alternative fuel for diesel combustion engines. DME is a gas at room temperature and pressure, but it will liquefy at about 5 bar. DME is also highly soluble in water. Aqueous solutions somewhat greater than 1.5 M DME can be prepared at ambient temperature and pressure. DME can be synthesized from natural gas with greater efficiency than methanol and thus it may be preferred over methanol as a fuel or fuel feedstock. DME can be made from methanol via an essentially irreversible reaction, whereas methanol is typically made via a reversible reaction step. Consequently, a higher yield of DME can be produced than methanol. Further, DME is relatively safe, especially compared to other common ethers.
DME has been used as a feedstock in producing reformate streams for use as fuel streams in fuel cell systems by external reforming. For instance, R. A. J. Dams et al. discuss the possibility of using reformed DME for solid polymer fuel cells in xe2x80x9cThe processing of alcohols, hydrocarbons and ethers to produce hydrogen for a PEMFC for transportation applicationsxe2x80x9d, Proc. Inter. Soc. Energy Convers. Eng. Conf. (1997), 32nd, p 837-842, Society of Automotive Engineers. Various apparatus and methods for reforming DME have been disclosed in the art, for example, in published European Patent No. 0754649 and PCT/International Publication No. WO 96/18573 (Application No. PCT/US95/15628). Furthermore, DME has been used as a fuel stream in solid oxide fuel cells, which typically operate circa 1000xc2x0 C. In this case, the DME is internally reformed in the fuel cell to produce molecular hydrogen and carbon oxides with the hydrogen, in turn, being oxidized at the anode.
Under certain conditions, it has been discovered that surprisingly good performance can be obtained from a fuel cell operating directly on dimethyl ether wherein dimethyl ether is directly oxidized to generate protons at the anode electrocatalyst. The operating temperature of the fuel cell is lower than that for internally reforming dimethyl ether to form molecular hydrogen. For instance, solid polymer fuel cells typically operate at temperatures less than about 200xc2x0 C., which is generally too low to internally reform dimethyl ether. Yet, direct dimethyl ether solid polymer fuel cells can exhibit satisfactory performance, particularly when compared to methanol performance in liquid feed solid polymer fuel cells. Thus, dimethyl ether is suitable for use as the primary fuel in a direct fuel cell system. Alternatively, since dimethyl ether has a desirably low freezing point, it may be used as a reactive antifreeze additive in the fuel supply of a liquid feed fuel cell, such as that of a direct methanol fuel cell.
In a direct dimethyl ether fuel cell, a fuel stream comprising dimethyl ether is supplied directly to the fuel cell anode for direct oxidation therein. Thus, a direct dimethyl ether fuel cell system comprises a system for supplying a dimethyl ether fuel stream to the anode. The fuel stream may contain other reactants and may desirably be supplied as a liquid. For instance, water is a reactant and the fuel stream may be an aqueous solution of dimethyl ether. Where the dimethyl ether is used as a primary fuel, concentrated solutions (about 1.5 moles of dimethyl ether per liter of water and up) may be employed. Or, the dimethyl ether may be used in combination with one or more additional fuels. For instance, the liquid fuel stream may comprise greater than about 0.1 mole of dimethyl ether per liter of water. The fuel stream may also optionally contain an acid.
The rate of reaction of dimethyl ether at the fuel cell anode may be significantly improved by employing higher fuel supply pressures. For instance, it can be advantageous to supply a liquid fuel stream to the anode at a pressure greater than about 4 bar absolute. The anode of the fuel cell may comprise a platinum ruthenium alloy electrocatalyst.
On the oxidant side, the performance of a direct dimethyl ether fuel cell may be satisfactory at a relatively low oxidant pressure, e.g. less than about 3 bar absolute. Further, the performance may be satisfactory at a relatively low oxidant stoichiometry, e.g. less than about 1.6. (Herein, stoichiometry is defined as the ratio of reactant supplied to that of reactant consumed.) The use of lower oxidant stoichiometries and/or pressures can be advantageous since supply of a compressed oxidant stream can represent a substantial parasitic load in a fuel cell system. Also, the startup of fuel cell systems from shutdown may often be delayed until compressors can supply an adequate supply of compressed air. Being able to operate at lower oxidant pressures can therefore accelerate the startup of such systems.
Particularly at low current densities, a direct dimethyl ether fuel cell may show efficiency advantages over other fuel cell types. For instance, an efficiency advantage may be obtained over direct methanol fuel cells particularly at current densities less than about 300 mA/cm2. Efficiencies are generally improved by recirculating unreacted dimethyl ether back into the fuel stream. Unreacted dimethyl ether is generally present in the anode exhaust, and it may also be present in the cathode exhaust as a result of crossover through the electrolyte. However, unlike certain other fuels like methanol, dimethyl ether does not typically react at the cathode electrocatalyst. Thus, any dimethyl ether fuel that crosses over to the cathode side is not consumed and need not be irreversibly lost. In principle therefore, unreacted dimethyl ether may be recirculated into the inlet fuel stream from both the cathode and anode exhausts.
A preferred system for directly supplying dimethyl ether in a fuel cell system may additionally comprise a mixing apparatus for providing the fuel stream for the fuel cell. Mixing apparatus inlets may then be fluidly connected to supplies of dimethyl ether and water reactant, while a mixing apparatus outlet may be fluidly connected to the anode of the fuel cell. If an additional fuel, such as methanol, is desired in the fuel stream, a supply of the additional fuel may also be fluidly connected to a mixing apparatus inlet. For various reasons, e.g. obtaining higher operating efficiencies under varying loads, it may be desirable to vary the composition of the fuel stream supplied to the anode during the operation of the fuel cell. In such a case, the mixing apparatus would desirably modify the composition in accordance with a suitable operating parameter of the fuel cell. To recirculate dimethyl ether from an electrode exhaust, a recirculation loop can be employed that fluidly connects the electrode exhaust to another mixing apparatus inlet. A heat exchanger may be employed in the recirculation loop to cool the fuel stream discharged from the electrode.
Having dimethyl ether in the fuel supply provides protection against freezing of a fuel cell system in general. However, introducing dimethyl ether into the cathode before shutdown is also beneficial in that freezing of water in the cathode during shutdown is prevented.